1. Field of the Invention
This invention relates to a room temperature setting adhesive composition capable of giving high initial bond strength and cured bonds having good water resistance and good boiling water resistance.
2. Description of the Prior Art
Emulsions based on polyvinyl acetate or vinyl acetate copolymers (hereinafter referred to as "PVAc emulsions") are in general use as room temperature setting adhesives. However, when used for bonding purposes, such PVAc-based emulsions give cured products or bonds poor in water resistance and in boiling water resistance, so that their use is limited to indoor applications. To remedy such disadvantage, adhesives comprising a PVAc emulsion, a resorcinol resin precondensate and formaldehyde or paraformaldehyde and adhesives of the same nature as above except that hexamethylenetetramine is used in lieu of said aldehyde have already been proposed.
However, the adhesives obtained by admixing a resorcinol resin precondensate and formaldehyde or paraformaldehyde with the above-mentioned PVAc emulsion have a strong formalin odor and this is of no small concern from the safety and hygiene viewpoint. Moreover, formalin released from such adhesives may offer various problems on the adherend side. As for the adhesives in which hexamethylenetetramine is used in lieu of the aldehyde and which can thus be regarded as improved forms, such problems due to the use of formalin as mentioned above can indeed be solved but said adhesives are still disadvantageous in that the rate of curing is very slow at room temperature, so that heating after spreading is required for their practical use. To remove such drawback, an adhesive has been proposed which comprises a vinyl acetate emulsion containing an acetoacetylated polyvinyl alcohol, a resorcinol resin precondensate and hexamethylenetetramine. This adhesive is advantageous in that it can be cured rapidly at room temperature and has very little formalin odor. Regrettably, however, the use of the above resorcinol resin precondensate in an amount not lower than a certain threshold level is essential for said adhesive to give cured products which are satisfactory with respect to water resistance, boiling water resistance, etc., but the use of the resorcinol resin precondensate in increased amounts disadvantageously causes a decrease in initial bond strength.